1. Field of the Invention
The invention relates to the field of automatic train protection, and in particular to a computer based method for implementing interlocking system logic.
2. Background Information
One of the greatest challenges in the development of a solid-state interlocking system for automatic train protection is to provide a system that can offer, at a minimum, the identical functionality that exists in a traditional, vital relay based system, while at the same time providing the flexibility and adaptability that is expected of modern-day computer based products. A "vital relay" or "gravity drop relay" is a relay having special characteristics which preclude welding of contact tips. It is mounted in such a way that upon de-energization of the relay coil, gravity will cause the contacts to open.
As the name implies, "automatic train protection" refers to an automatic system for protecting trains traversing a transportation network, automatically avoiding guideway conflicts which could lead to train collisions, and ideally at the same time optimizing guideway utilization and overall train system efficiency. The term guideway refers to the track which guides a train between points A and B. Guideway "objects" are the switches, turntables, scissors cross tracks, etc., through which a train travels on its journey.
The term "interlocking system" as used herein refers to an arrangement of gates and control apparatuses interconnected so that their functions must occur in a predetermined sequence to assure safety. A gate is the boundary point of an interlocked route where entry to a route is governed. A gate is not a device, but rather a point on the guideway.
Some of the current solid-state interlocking systems which have been developed merely attempt to supplant the vital relays with the boolean logic equations representative of the interactions that would occur between the relays as they transition from one state to another.
This type of solution represents an improvement in that it eliminates the expense of the actual relays themselves, their maintenance, etc. However, it does not eliminate the additional steps of having an interlocking engineer design the interlocking system, transform it into representative boolean equations, have it verified by an interlocking design inspector, etc., expensive processes in their own right. In addition, it does not provide the desired flexibility, so that any changes or adaptations made after the fact require the entire process to be repeated.
With the advent of powerful microprocessor technology, several attempts have been made by members of the railroad industry to replace relay-based interlocking systems with solid-state microprocessor-based systems. The more flexible of these systems have, for the most part, performed this function by solving boolean equations or ladder logic representative of the relay-tree diagram created by an interlocking designer.
Some of the problems with this approach are errors in the interlocking logic are difficult to detect, and future changes to the interlocking (e.g., guideway extensions, addition of B-point detectors, etc.) must be carefully implemented into the overall interlocking scheme by an experienced interlocking engineer.
To further complicate matters, since automatic train protection systems are safety-related, any changes to the code executed by the microprocessor usually requires at least a partial re-certification of the system, resulting in a longer system down-time, not to mention other costs associated with obtaining safety certification.